Proper choice of adhesive

Proper Choice of Adhesive Good Joint Design Cleanliness Wetting... 

As previously mentioned, some urethanes can biodegrade easily by hydrolysis, while others are very resistant to hydrolysis. The purpose of this section is to provide some guidelines to aid the scientist in designing the desired hydrolytic stability of the urethane adhesive. For hydrolysis of a urethane to occur, water must diffuse into the bulk polymer, followed by hydrolysis of the weak link within the urethane adhesive. The two most common sites of attack are the urethane soft segment (polyol) and/or the urethane linkages. Urethanes made from PPG polyols, PTMEG, and poly(butadiene) polyols all have a backbone inherently resistant to hydrolysis. They are usually the first choice for adhesives that will be exposed to moisture. Polyester polyols and polycarbonates may be prone to hydrolytic attack, but this problem can be controlled to some degree by the proper choice of polyol. 

For the production of wood-based panels various adhesives are in use like aminoplastic resins (UF, MU(P)F), phenolic resins (PF) or isocyanate (PMDI). The proper choice of the adhesive depends on the required properties of the wood-based panels, on the working conditions during the production as well as often on the costs for the adhesive system this not only means the net price of the adhesive but the overall costs of the gluing system including glue spread factor. 

Resist adhesion must be high enough so that the film does not peel off during development. Trapped voids, either in the bulk or at the wafer interface, must be removed completely. Trapped voids in the bulk are eliminated by proper choice of solvent, which must evaporate slowly to avoid... 

For barrier coatings to be effective it is necessary to have (i) good adhesion of the barrier film on the metal surface (this is generally accomplished by multicoat procedure to a certain extent) (ii) proper choice of the polymers to control the permeability of oxygen and (iii) optimum film thickness achieved by multicoats or thin-film system, depending upon the envisaged application. 

Proper choice of filler allows to modify shrinkage of UV curable transparent adhesives.The shrinkage of UV curable adhesive was reduced by factor 2-3 by incorporation of quartz which has better UV light transparency than conventionally... 

By proper choice of either the isocyanate or the polyol, actual chemical cross-links can be introduced in either the hard or soft segments that may be beneficial to some properties. The effectiveness of these cross-links is offset by a disruption of the hydrogen bonding between polymer chains. Highly cross-linked polyurethanes are essentially amorphous in character exhibiting high modulus, hardness, and few elastomeric properties. Many adhesives fall into this category. 

A review of the traditional use of resins as tackif iers for elastomers in solvent-applied adhesives is presented. Following is a discussion of how this technology is extended to water-based systems. Data is discussed on the use of various backbone polymer latexes, modified with the proper resin emulsions to achieve pressure—sensitive adhesive properties. Recommendations are also given for the proper choice of resin type to achieve optimum properties with each specific class of elastomer. 

Other properties that can be affected with the proper choice of fillers for a specific application include compressive strength, adhesion, arc and tracking resistance, density, and self-lubricating properties. 

A wide variety of plasticizers are suitable for PVB and PVF resins. For many years, the universally used plasticizer for PVB was triethylene glycol di(2-ethylbutyrate) [95-08-9] (6). More recently this has been supplanted by triethylene glycol di(2-ethylhexanoate) [94-28-0], tetraethylene glycol diheptanoate [70729-68-9], dihexyl adipate [110-33-8], 2-ethylhexyl diphenyl phosphate [1241-94-7], and a variety of other oligomeric ethylene glycol esters and ethers, and other adipate, phosphate, phthalate, sebacate, and ricinoleate esters (19,20,57-62). For PVF, diethyl, diphenyl, and dicyclohexyl phthalates, as well as tributyl, triphenyl, and tricresyl phosphates are useful plasticizers (21). By proper choice of plasticizer type and level, the physical-mechanical, chemical, and adhesion properties of these resins can be tailored for a wide variety of applications (see Plasticizers). 

Superfast curing (in less than 0.5 s) of acrylate adhesives for plastics has been achieved by a proper choice of prepolymers, photoinitiators and photosensitisers, and successfully used in blister packaging [81], as well as for bonding glass to plastics [78]. 

Rubber/cement (RC) failures indicate that the weakest point in the bonded part is at the interface between the rubber and adhesive. These failures are characterised by a relatively glossy and hard bonded surface with little or no rubber present. Common causes of RC failure are the incorrect choice of adhesive, insufficient dry film thickness of adhesive, failure to properly agitate the adhesive to achieve a uniform dispersion prior to application, pre-cure of adhesive caused by excessive dwell time in the mould cavity before introducing rubber, low mould pressure, undercure of the part, migration of plasticisers and other ingredients from the body of the rubber to the rubber/adhesive interface or contamination of the surface of the adhesive coated part. 

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